1. Field of the Invention
The present invention relates to a toner, and a process cartridge and an image forming apparatus using the toner.
2. Discussion of the Background
Toner for use in forming images by electrophotography, etc. is typically a heat fusion powder in which a coloring agent is dispersed in a natural or synthetic resin. Therefore, the toner image transferred to a recording medium after development is fused and fixed thereon upon application of heat and pressure.
A heat roller system is widely used as the fixing system for use in dry development in terms of the energy efficiency. Recently, the heat energy provided to the toner during fixing has been decreasing by using a toner having a low temperature fixing property to save energy. Technology Procurement Project for the next generation photocopier was set in the DSM (demand-side Management) programs of IEA (International Energy Agency) in 1999. The requirement by the Project was published and it said that drastic energy saving should be achieved in comparison with typical photocopiers. To be specific, with regard to a photocopier having a speed of 30 cpm or higher, the waiting time is within 10 seconds and the consumption energy during stand-by is from 10 to 30 Watt (depending on the photocopying speed). Therefore, lowering the toner fixing temperature during actual use by lowering the fixing temperature of toner is considered to be an unavoidable technology to satisfy the requirement and minimize the waiting time. In an attempt to deal with the technology issue of lowering the fixing temperature of toner, a polyester resin has been used instead of a styrene acryl based resin, which has been diffused as a binder resin for toner, because the polyester resin has an excellent low temperature fixing property and mechanical strength while also having a relatively good high temperature preservability.
For example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2002-351143 describes a technology in which toner particles are granulated by dissolving a polyester having an end modified by an isocyanate group and an unmodified polyester in an organic solvent to obtain an oil phase and adding a diamine compound when dispersing the oil phase in an aqueous dispersion to conduct isocyanate elongation reaction. In this method, mechanical strength is imparted to the toner by a cross-linked structure formed due to urea linkage of the isocyanate modified polyester. In addition, by designing the unmodified polyester to have a relatively low molecular weight, impregnation of toner on paper during fixing is accelerated, which leads to amelioration of the low temperature fixing property of the toner.
However, a single component toner formed of the toner manufactured as described above easily causes problems of production of abnormal images having streaks, uneven density, background fouling, etc. especially when images are continuously output because the toner particles tend to be insufficiently charged and crack or deform under the pressure by a toner regulation member, resulting in attachment of the toner to the toner regulation member or deterioration of chargeability. The mechanism of these problems is not clear but it is inferred that since the elongation agent such as the diamine compound to conduct elongation reaction has a high polarity, the elongation agent tends to detach into the aqueous phase or move to the interface between the oil phase and the aqueous phase. Therefore, the elongation reaction proceeds unevenly and insufficiently so that the toner does not have a strong mechanical strength. Furthermore, the elongation agent such as the diamine compound which tends to have a significant impact on the chargeability is present around the surface of the toner particles. Therefore, the toner is placed under mechanical stress, which deforms the toner surface. When the inside of such deformed toner is exposed, the toner is not uniformly charged because such deformed toner particles have different chargeability from other toner particles. This is another thinkable cause for the background fouling.